Sunday, November 8, 2015

The origin of large body size and herbivory in giant Canary Island lacertids

Gallotia galloti. Photographer's Credit: Petermann
2005, via Wikimedia Commons
The Island Rule on body size claims big animals  evolve smaller body sizes due to the limited resource of food and limited habitat; while smaller animals that have no natural enemies in islands evolve larger body sizes.

 Andrej Čerňanský and colleagues discovered a fossil related to the genus Gallotia which is endemic to the  Canary Islands. They named it Janosikia ulmensis, after the Slovak national hero, outlaw Juraj Jánošík. Zoological Journal of the Linnean Society reports the results. The genus Gallotia has not been growing in size on the island – quite the contrary. The Gallotia ancestor came to the island with a large body size.

Until now, the genus Gallotia was considered a clear example of island gigantism; scientists supposed that a small lizard resembling the North African species Psammodromus colonised the Canary islands 20 to 18 million years ago. As it did not have any natural enemies, it supposedly gradually increased its body size several times. But the fossil record illustrating this  was absent.

“Currently, we have been able to do research, for the first time, on fossils and almost complete findings from about 22 million years old which are in the line leading to the lizards from the Canary Islands. Our fossil comes from Germany and precedes the period of the islands’ colonisation. It also shows us a completely different story, proving that the evolution of body size is much more complex than we had originally thought,” Čerňanský explained.

This stems from a whole set of anatomic features – like the huge size, as the skull alone was almost five centimetres long, the Slovak researcher said. This shows that this line had already grown to this size on the continent before colonising the islands; some other current species from the islands, on the other hand, represent markedly smaller animals. Thus, Janosikia turned the original idea upside down.

The fossil is crucial for understanding the island rule of evolution of body size, and also the herbivory of today’s large species of lizards. It also confirms the assumption of molecular biologists that the line leading to the genus Gallotia has its origin on the European continent; and, last but not least, it is one of the best preserved lizards from the Tertiary that reveals many important, so far unknown aspects of the evolution of dominant group of reptiles in Europe – the family Lacertidae, Čerňanský summed up.

Janosikia ulmensis is from the early Miocene of Ulm, Germany (∼22 Mya). The authors show that this species and the Oligocene Pseudeumeces cadurcensis (Filhol, 1877) are in fact crown lacertids, and the first known pre-Quaternary record of the total clade of Gallotia. Pseudeumeces confirms the early origin of crown Lacertidae in the Palaeogene of Europe. More importantly, these fossil taxa show that large body size was already achieved on the European mainland by the early Miocene. Furthermore, Pseudeumeces and Janosikia were faunivorous, thus demonstrating that insularity, not large body size, was crucial to the evolution of herbivory in this lineage. Body size change in Gallotia was more complex than previously thought, encompassing size increase [e.g. in the extinct Gallotia goliath (Mertens, 1942)], but more commonly involving miniaturization. The physical environment may play a crucial role in modulating the evolution of body size.

Čerňanský, A., Klembara, J. and Smith, K. T. (2015), Fossil lizard from central Europe resolves the origin of large body size and herbivory in giant Canary Island lacertids. Zoological Journal of the Linnean Society. doi: 10.1111/zoj.12340.



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